Cyclic adsorption processes are well know and are typically used to separate a more absorbable component gas from a less absorbable component gas. Examples include pressure swing adsorption (PSA) or vacuum pressure swing adsorption (VPSA) processes which use a low pressure or a vacuum and a purge gas to regenerate the sorbent and temperature swing adsorption (TSA) processes which uses a thermal driving force such as a heated purge gas to desorb the impurities. Such processes are generally used to separate oxygen or nitrogen from air, other impurities like hydrocarbons and/or water vapor from feed air gases, hydrogen from carbon monoxide, carbon oxides from other gas mixtures, and the like. These processes are also used to remove impurities such as water vapor and hydrocarbons from air prior to cryogenic air separation. Any cyclic adsorption system for separating or purifying gas components can be used.
For illustrative purposes, a typical VPSA process for separating oxygen from air is described herein although the present invention can be employed with other cyclic adsorption processes using centrifugal compressors and is not intended to be limited to this process. The typical cyclic VPSA process is one wherein an adsorber bed undergoes the following stages:
1. The adsorber bed is pressurized to a desired pressure wherein nitrogen is readily adsorbed by the adsorbent as the feed air is passed across the bed;
2. Product gas rich in oxygen is produced as the nitrogen in the feed air is adsorbed;
3. The bed containing the adsorbent is evacuated to a low pressure (typically under vacuum) wherein the adsorbed nitrogen is desorbed from the adsorbent in the adsorber bed; and, preferably,
4. A purge gas is passed through the bed to remove any remaining nitrogen.
The cycle time is understood by the skilled person to mean the amount of time needed to complete one cycle; e.g. the process steps in order and then return to the starting condition.
Some adsorption processes will have more steps or multiple beds and often use one or more blowers for each of the pressurization and depressurization steps. If the VPSA plant contains two or more adsorber vessels, each vessel undergoes the above steps; however, the two vessels are operated out of phase so that while one vessel is producing product the other is being regenerated. Also, in a two bed process two blowers are typically used wherein one is dedicated to feeding gas to the adsorber vessels while the other dedicated to evacuating the adsorber vessels.
Regardless of whether a single vessel, two vessels, or even more vessels are used, the pressures and flows within the process change quickly as the process cycles from adsorption to desorption. Generally, the pressure of a vessel will change from a low pressure condition of at or below atmospheric, preferably below atmospheric , such as about 6 to 8 psia, to a high pressure condition of above atmospheric, such as about 19 to 24 psia, in a rapid periodic cycle time, such as less than one minute. Some adsorption processes will require even wider spans of pressures and/or vacuums in similar rapid cycle times.
Traditionally, VPSA plants use positive displacement machines such as rotary lobe type blowers operating at fixed speeds to move gas through the process. These machines are robust and generally do not experience any significant operational problems as the pressures and flows change and reverse. However, these machines have low power efficiency and conventional machines are only 60-65% efficient. About 35-40% of the energy supplied to these machines is therefore wasted. Thus, it is clearly desirable to replace the traditional rotary lobe machine with a more efficient machine capable of meeting the rigorous requirements of rapid cyclic conditions.
One such machine is a centrifugal compressor driven by a direct drive variable, high speed permanent magnet motor or a variable, high speed induction motor. Such compressors have a known efficiency of approximately 85%. The challenge involved in the use of such a compressor is that its performance is very sensitive to changes in pressure, such as the rapid pressure changes that occur during a pressure swing adsorption process. Centrifugal compressors used in rapid cyclic processes like the adsorption process described herein are highly susceptible to frequent adverse operating conditions or states known as surge and stonewall as are more fully described below. Such conditions can result in both low power efficiency and damage or failure to the compressor impeller and other compressor or system components and have therefore been avoided in practice. Thus, it is necessary to manage the adverse conditions of surge and stonewall if one is to succeed in replacing positive displacement machines with more efficient centrifugal machines in cyclic adsorption processes.
Centrifugal compressors have been proposed for use in adsorption processes. For example, U.S. Pat. No. 5,555,749 suggests the use of centrifugal compressors in adsorption systems during the exhaust portion (depressurization) of the cycle. U.S. Pat. No. 7,785,405B2 discloses systems and processes for gas separation using high-speed permanent magnet variable-speed motors to accelerate and decelerate centrifugal compressors used in pressure swing adsorption (PSA) or vacuum pressure swing adsorption (VPSA) processes. These patents do not teach a process in which the optimal cycle time is selected to realize the power benefits from the use of such compressors.